1. Field of the Invention
The invention relates to a resin composition for being extruded on an outer periphery of a conductor so as to form a foam insulation, and a foam insulated wire using the same.
2. Related Art
In recent years, portable communication devices as typified by a cellular phone have been remarkably developed, and devices/components used for telecommunication thereof, such as antenna, wire or cable, are strongly required to have high performance.
Wires/cables used for an antenna of a communication device is required to reduce mainly transmission loss. Factors greatly affecting on the transmission loss are mainly a thickness of a wire/cable (conductor) and electrical characteristics of an insulator. If it is possible to reduce the transmission loss by improving these factors, a thinner conductor can be used while the transmission loss is still the same, and it is possible to achieve significant resource-saving and cost reduction.
Among the electrical characteristics of the insulator, factors affecting on the transmission loss are dielectric loss tangent and permittivity, which are reduced by foaming polyethylene in a currently popular method.
As a method of manufacturing a foam insulated wire, there are roughly two types of manufacturing methods as below.
Physical Foaming Method
One of the methods is to inject high pressure gas into resin melted in an extruder, which is called a physical foaming method. Here is an outline of the procedure.    (1) Resin is introduced into an extruder and is melted by kneading under heat.    (2) High pressure gas is injected at the middle of resin flow path so as to be dissolved in the resin.    (3) A conductor is coated with the resin having the gas dissolved therein.    (4) The resin applied is moved outside the extruder in accordance with the movement of the conductor.    (5) Pressure inside the extruder is released to make air-bubbles from the gas dissolved in the resin.    (6) The resin is cooled and solidified before excessive growth of the air-bubbles which causes non-uniformity in an insulator.
Chemical Foaming Method
Another method is to introduce a chemical foaming agent together with a resin, which is called a chemical foaming method. Here is an outline of the procedure.    (1) Resin and a foaming agent are introduced into an extruder. The foaming agent may be kneaded alone or with the resin.    (2) It is heated in the extruder to a decomposition temperature of the foaming agent or more. At this time, gas generated is dissolved in the resin while the high pressure state of the resin is maintained so that foaming does not occur in the extruder.    (3) A conductor is coated with the resin having the gas dissolved therein.    (4) The resin applied is moved outside the extruder in accordance with the movement of the conductor.    (5) Pressure inside the extruder is released to make air-bubbles from the gas dissolved in the resin.    (6) The resin is cooled and solidified before excessive growth of the air-bubbles which causes non-uniformity in an insulator.
In comparison to the chemical foaming method, the physical foaming method has following advantages.    (1) High foaming rate is easily obtained.    (2) Since a chemical foaming agent is not used, deterioration in electrical characteristics (permittivity or dielectric loss tangent) of an insulator caused by the foaming agent or a residue thereof less occurs.
That is why the physical foaming method is often used for manufacturing a high-performance foam insulated wire.